Thermal vibration contribution to continuum stress in the elastic regime

Homogeneous deformation of an ordered crystalline solid at finite temperature can cause non-affine transformation of atomic trajectories. In such a case, continuum measures based on affine transformation of trajectories are insufficient to ensure energetic equivalence between the atomic and continuum scales. We use molecular dynamics simulation of fcc aluminum in NVT ensembles to demonstrate that the second moments of atomic positions about equilibrium show increasing deviation from affine behavior with strain and temperature. While the Cauchy-Born rule enforces affine deformation of the crystal in the static sense, second moments have been used to quantify non-affinity in thermal vibrations due to atomic trajectory transformation under macroscopic deformation. The evolution of second moments with applied strain becomes highly non-linear at high temperatures in the tensile regime as a result of the anharmonic potential energy surface of the material. The vibrational entropy is computed using the time-averaged second moments of position from the molecular dynamics simulation. Stress computed using the resulting free energy is significantly lower than the virial stress in the high tensile strain – high temperature regimes

Chemo-mechanical characterization of phase-separated dentin adhesives

The premature failure of composite dentin restorations can be traced to the adhesive-dentin (a/d) interface. The continuity, stability and mechanical properties of the dentin adhesive polymer in the interface depend on its composition and morphology. The adhesive is composed of a hydrophilic/hydrophobic monomer pair, which phase separates upon interaction with dentinal water, resulting in variable chemical composition and pore structure of the adhesive in the interface. The phase separation has been quantified using a ternary monomer-water phase diagram. The polymerization of the monomer phases, composition and pore structure of the resulting polymer phases were investigated. A novel micro-poromechanical constitutive model was developed to model the polymer mechanical, swelling and transport behavior while accounting for polymer-water interaction and pore pressure. The model was then used to characterize polymer formulations along the phase boundary. Parametric studies were performed to illustrate flow-dependent mechanical behavior of hydrophilic polymers. Experimental observations showed negligible amount of crosslinker in hydrophilic-rich phases and identified the cut-off point on the phase boundary to achieve significant polymerization. The results also quantified the composition of polymers formed along the phase boundary. Crosslink density and hydrophilicity were identified as the critical parameters which determined the sorption, swelling, diffusion and poromechanical behavior of the polymer phases. These two parameters were found to depend on the phase chemical composition. Effects of external environment and confinement on the flow-dependent mechanical behavior have also been studied parametrically for hydrophilic polymers. The combination of experimental results and micro-poromechanical modeling provide structure-property relationships obtained from first principles, which will aid in adhesive design and analysis of the adhesive-dentin interface performance

C-H...O interactions and stacking of water molecules between pyrimidine bases in 5-nitro-1-([beta]-D-ribosyluronic acid)-uracil monohydrate [1-(5-nitro-2,4-dioxopyrimidinyl)-[beta]-D-ribofuranoic acid monohydrate]: a neutron diffraction study at 80 K

This is the publisher's version, also available electronically from http://scripts.iucr.org/cgi-bin/paper?S0567740879006506.See article for abstract.Research carried out at Brookhaven National Laboratory under contract with the US Department of Energy, and supported by its Office of Basic Energy Sciences

Swelling equilibrium of dentin adhesive polymers formed on the water-adhesive phase boundary: Experiments and micromechanical model

During their application to the wet, oral environment, dentin adhesives can experience phase separation and composition change which can compromise the quality of the hybrid layer formed at the dentin-adhesive interface. The chemical composition of polymer phases formed in the hybrid layer can be represented using a ternary water-adhesive phase diagram. In this paper, these polymer phases have been characterized using a suite of mechanical tests and swelling experiments. The experimental results were evaluated using granular micromechanics based model that incorporates poro-mechanical effects and polymer-solvent thermodynamics. The variation of the model parameters and model-predicted polymer properties has been studied as a function of composition along the phase boundary. The resulting structure-property correlations provide insight into interactions occurring at the molecular level in the saturated polymer system. These correlations can be used for modeling the mechanical behavior of hybrid layer, and are expected to aid in the design and improvement of water-compatible dentin adhesive polymers

Fifty Years of ISCA: A data-driven retrospective on key trends

Computer Architecture, broadly, involves optimizing hardware and software for current and future processing systems. Although there are several other top venues to publish Computer Architecture research, including ASPLOS, HPCA, and MICRO, ISCA (the International Symposium on Computer Architecture) is one of the oldest, longest running, and most prestigious venues for publishing Computer Architecture research. Since 1973, except for 1975, ISCA has been organized annually. Accordingly, this year will be the 50th year of ISCA. Thus, we set out to analyze the past 50 years of ISCA to understand who and what has been driving and innovating computing systems thus far. Our analysis identifies several interesting trends that reflect how ISCA, and Computer Architecture in general, has grown and evolved in the past 50 years, including minicomputers, general-purpose uniprocessor CPUs, multiprocessor and multi-core CPUs, general-purpose GPUs, and accelerators.Comment: 17 pages, 11 figure